A Defense of Those Who Defended Ptolemy

We have all heard the story of how Copernicus changed the world with his Heliocentric model of the solar system. It's an especially memorable story because of all the drama that went along with the change. Copernicus himself was so afraid of what other people would do in response to his ideas that he didn't allow them to be published until after his death. Everyone knows the story of how Galileo, a brilliant advocate of the Copernican model, was persecuted by the Catholic Church for his beliefs, and was eventually forced to recant.

This is a great story because it presents us with a bunch of underdogs, the Copernicans, and a big bully of a bad guy, the Church. It's a tragic tale too, because so many great men didn't get the honors they deserved in their lifetime. The story serves a valuable purpose, in reminding us that we should listen to science, that science is the great creator of knowledge in a modern world, and that we shouldn't let the establishment crush good ideas simply because it disagrees with them.

One thing that often happens in these stories is that the Church's geocentric model is mocked as dumb, or unnecessarily complex, or as being an article of faith that has nothing to do with reality. This is an idea that I think we should revise. The first thing to realize is that an analysis of the solar system based on a geocentric model was a well-established science at the time of Copernicus, and had been fairly successful. The success of the geocentric model came from a Greek guy who died around 168 AD, known to us as Ptolemy.

Before Ptolemy, we had a conception of the heavens as a sphere around Earth, which spun about us once a day. This model was known to be insufficient though, because it failed to account for these strange stars that the Greeks called the Planets, which translates as “wanderers.” They were called wanderers because they were never in the same place each night, always moving around, sometimes right next to one star then next to another. The weirdest thing is that they seemed to advance in one direction for a while, then all of a sudden switch and go backwards, then return to normal motion a few weeks later.

In Ptolemy's time, there were conceptions that the planets were special types of bodies, different from stars in certain ways. Ptolemy, however, gave people a really good way of understanding what planets are and why they behave the way they do. He proposed that the planets move in circles around the Earth, but also move in circles around an invisible point. Strictly speaking, it is the invisible point that circles the Earth, not the planet itself. The model is easiest to describe with a picture:

[http://www-personal.ksu.edu/~lyman/english233/cosmos4.htm]

Note also that Ptolemy conceived of everything here being tied to the celestial sphere, which rotates around the Earth once per day in addition to all the other slower motion going on.

This model explains rather elegantly why planets appear to reverse direction for a time, then return to normal motion. The planets are reversing when they are in the part of their circle closest to Earth, then returning to normal motion as they circle around away from Earth. This model proved to be reasonably effective at predicting the location of planets in the sky. It wasn't perfect, but good enough for most purposes.

As time went on, it was found more and more to be the case that although the planets appeared roughly where Ptolemy predicted, they were not in the exact place he predicted. Astronomers realized that they could more accurately predict the location of the planets by postulating additional circles, so that planets were modeled as moving in a circle along a circle going along another circle. By the time of Copernicus, they were postulating up to 16 circles in a single model of the motion of the planets.

To us, this model seems pretty goofy. It also seems obvious why the Copernican model is far superior. A heliocentric model explains the motion of the planets quite nicely: the planets move along in the sky as they orbit the sun, but every once in a while either they pass us or we pass them, and they go the other way in the sky. It does this without having to resort to any sort of circles in circles, and greatly reduces the complexity of the system. Scientists like to appeal to the principle of Occam's Razor, the idea that one should not unnecessarily postulate entities where fewer entities would suffice. Occam's Razor seems to strongly suggest a heliocentric model.

However, the model Copernicus gave to the world is not a complete model at all. First off, Copernicus too had to put in some circles within circles to make his model match observation. We know today that the planets don't move in circles at all, but instead move in ellipses, but nobody would realize this until Kepler, well after the death of Copernicus. The Copernican model was no more predictive than the Ptolemaic model, and seemed equally arbitrary.

More importantly, the heliocentric model of the solar system implicitly rejects certain important metaphysical conceptions of the time. The scientists of the Middle Ages and Renaissance had a working understanding of why things fell. They conceived of the world as being made of Aristotle's four elements: earth, wind, fire, and water. Things made of fire naturally rose to the very top of the atmosphere, where they surrounded the earth in a ring of fire (this explains, for instance, why hot air rises). Things made of air naturally came second, rising up and filling the atmosphere. Water naturally came towards the center of the earth, but sat on top of the element earth. Earth naturally wanted to compact itself as closely to the center of things as possible. This worldview had a fair amount of explanatory power. They could explain why certain things floated while other things didn't, they understood how a round earth made sense without things falling off the bottom; it was, overall, a useful way of understanding the physical world. This conception also rejected the idea that the heavens were made of the same stuff as us. They proposed a fifth element, whose natural place was the heavens. Obviously the heavens could not be made of the four normal elements, because otherwise they would be down here seeking their proper place, like all the other things made of those elements.

None of this makes any sense if the earth is rapidly moving, flying through space at a thousand miles an hour. Earth is, as astronomer Tycho Brahe put it, a “lazy element.” It is clearly not in earth's nature to move without a significant force acting upon it making it move. And if the earth is constantly moving, why do we not notice its movement? Today, Newton's laws of motion explain why we don't notice earth's motion. Newton's laws are powerfully counter-intuitive though. Our experience teaches us that an object we push will move until we stop pushing, then will slow down and stop. Science-fiction space battles routinely show us ships with disabled engines slowing down and stopping, the way things work in our normal experience. No force means no movement. Newton actually proposes that the stopping of motion is itself a second force, the force of friction. Doesn't this violate Occam's Razor though? He proposes two forces acting on every object we have ever observed behaving in a totally normal way, when its much more easily explained as just one force that's either acting or not.

My point is not that Newton is wrong. It is that Occam's Razor is a poor criterion for choosing between two theories. Copernicus may have made astronomy simpler, but he made the rest of physics a hell of a lot more complicated. People who rejected the Copernican model were not just fools who refused to see what was obviously true, they were people who were unwilling to scrap an entire metaphysical system to make their model of the cosmos a little easier to draw. It is significant that Copernicanism wasn't widely accepted until people like Galileo and finally Newton created new rules of physical motion that replaced the old Aristotelian system. Copernicanism is not truly sensible until you've postulated all sorts of other complicated, counter-intuitive concepts.

This is why we shouldn't look too harshly on those who defended Ptolemy. Certainly, silencing someone for their views goes against good practice, but it becomes a lot more understandable when their views undermine not just our vision of the cosmos, but our vision of how the world itself works. And most people who argued against heliocentrism didn't use force to make their arguments, they used argumentation, and their argumentation was based on a well-established explanatory theory, not based on pure faith or a blind denial of reality.